1) Field of the Invention
This invention relates to electrical transformers, and more particularly to electrical transformers, that are arranged about an electrically conductive member and surrounded by an electrically conductive housing, as is typically found with transformers employed in down-hole drilling equipment used for the exploration of oil and gas.
2) Description of the Prior Art
Modern drilling techniques employ an increasing number of sensors in down-hole tools to determine down-hole conditions and parameters as pressure, spatial orientation, temperature, gamma ray count etc. encountered during drilling. These sensors are usually employed in a process called xe2x80x98measurement while drillingxe2x80x99 (MWD). The data from such sensors is either transferred to a telemetry device, and thence up-hole to the surface, or is recorded in a memory device by xe2x80x9cloggingxe2x80x9d
The oil and gas industry presently uses either a wire (Wireline), pressure pulses (Mud Pulse) or electromagnetic (EM) signals to telemeter all or part of this information to the surface in an effort to achieve near real-time data. Another telemetry technique is starting to be developed and deployed, namely acoustic telemetry (AT). This technique generally depends on driving a piezoelectric or magnetostrictive element (transducer) via a battery-powered source in order to produce acoustic waves that travel along the drill string, conveying drilling information.
The piezoelectric or magnetostrictive transducer devices normally require a high voltage or large current source respectively, and this is normally delivered from a battery via a transformer and associated electronic circuitry. To make the most efficient use of the annular space, the transformer is wound on a toroidal core and is axially engaged on the inner housing, then covered and protected by the outer housing. Insertion may require the toroidal core to be assembled from two halves. The tubular outer housings must resist tension, compression, torsion, bending, shock and vibration, high pressure and high temperatures in a typically harsh drilling environment. To be adequately strong they are almost always made from steel, titanium or beryllium copper.
A consequence of this choice of materials is that they form excellent electrical conductors. In order to prevent the high pressure drilling fluid and other materials from entering the annular cavities and interfering with the correct working of internal assemblies and components, the inner and outer housings are variously connected with xe2x80x98o-ringxe2x80x99 seals to render the annular space pressure-proof. This requirement in almost all cases also provides an uninterrupted electrical connection between inner and outer housings at each end of the protected annular spaces.
More specifically, a simple downhole housing that contains an axially engaged toroidal transformer in its annular space often forms an electrical short circuit turn around and through the transformer. This shorted turn has a deleterious effect on transformer performance and is conventionally dealt with by inserting a non-conductive material such as a ceramic disc into the housing, thereby electrically opening the shorted turn. (e.g. refer to U.S. Pat. Nos. 6,249,259 B1 and 4,691,203). For instance in a design specifically for a hollow core magnetic dipole, U.S. Pat. No. 6,249,259 B1 shows how to split a conductive sleeve associated with a transformer in a manner that prevents a closed loop from being formed. U.S. Pat. No. 4,691,203 similarly teaches the use of an insulative gap useful in a drill stern/earth telemetry application.
This mechanical solution can lead to mechanical design complications and lessens the robustness of the transformer housing. It is advantageous to remove the need to insert a non-conductive break, particularly in the extremely harsh environment associated with downhole drilling. From an electrical perspective the toroidal transformer is threaded by a low resistance shorted turn, enabling a large current to flow through the housings. This causes the transformer to suffer a significant loss of efficiency. A conventional solution to avoid this issue is to create an electrical break in the current loop, generally implemented by inserting a non-conductive ring. This solution adds complexity and will make the assembly less robust, particularly in the harsh environment associated with drilling. Our invention eliminates the need to interpose a non-conductive ring, thereby maximizing the mechanical reliability of the housing assemblies.
It is an object of the invention to overcome the deleterious and unintended effects of a shorted turn on transformer applications, particularly for inclusion in tools designed for oil and gas exploration where space to put subassemblies such as transformers is at a premium. It is well known in our industry that incorporating an electrical discontinuity or a significant resistive component can prevent shorted turn effects. By a means novel in our industry we accomplish the objective without incorporating a discontinuity or similar resistive means, thus avoiding the need for extra mechanical modifications and avoiding potential structural weakening of housings containing such transformers.
Our invention recognizes that a transformer works by coupling a primary winding to a secondary winding by a time-varying magnetic field. This magnetic field is generally confined to the core, but fields generated external to the transformer associated with current in the windings couple into the inner and outer metal tubular housings and generate eddy currents. These eddy currents combine in concert when a continuous electrical path is available and cause the shorted turn large current effect. Our invention is to accept this effect in order to maintain a simpler and more robust mechanical enclosure around the transformer, but mitigate it by providing a second transformer and inducing an equal but opposite current derived from a second similar transformer disposed on and within the same tubular housings as the first. The net effect is that each transformer, in generating its own shorted turn current, ideally negates that of the other, leaving no net current flow in the shorted turn conductor path.
The output of each transformer may be connected such that they combine to drive a common load or even a split load. The important issue is that the transformers are substantially balanced in operation such that they generate equal (or nearly so) but opposite shorted turn currents.
Accordingly, in a broad aspect of our invention, such invention comprises a pair of transformers, each substantially similar-sized and substantially housed in electrically-conductive outer housing means, said pair of transformers located in close proximity to each other within said outer housing means, each transformer comprising at least a primary winding and a secondary winding,
(a) each of said primary and secondary windings disposed about an inner electrically conductive member;
(b) each of said pair of transformers, when a current is passed through said primary winding thereof, inducing a respective eddy current; and
(c) said primary winding of each transformer electrically connected to the primary winding of the other transformer so that the induced eddy current of one of said transformers is substantially equal but opposite in direction to the induced eddy current of the other of said transformers so as to substantially cancel each other out.
In another broad aspect of the invention, such invention comprises a pair of transformers, substantially housed in electrically-conductive outer housing means, said pair of transformers located in close proximity to each other within said outer housing means, each transformer comprising at least a primary winding and a secondary winding,
(a) each of said primary and secondary windings disposed about an inner electrically conductive member in electrical communication with said outer housing means; and
(b) said primary winding of a first of said pair of transformers electrically connected to the primary winding of the other of said pair of transformers so that electrical current, when caused to pass in the primary winding of the first transformer, runs in a direction opposite to electrical current in the primary winding of the other transformer.
The invention allows eddy currents to be harmlessly generated in the shorted turn, as eddy currents generated by one primary winding of the first transformer are substantially opposed in magnitude and sense by eddy currents induced by opposite current flow in the primary winding of the other transformer. By xe2x80x9csplittingxe2x80x9d the original transformer into two or more transformers, and connecting the windings thereof such that the eddy currents generated in the shorted turn formed by the housings are substantially split into equal and opposing currents, thereby nullifying any net significant current flow, the advantage is that close to the previous transformer efficiency can be realized.
Advantageously, this result is achieved in almost the same annular space as a single transformer, without having to insert an electrically non-conductive member to prevent the shorted turn from reducing the transformer efficiency, thereby maintaining structural integrity of the outer housing which is necessary in down-hole drilling environments in which these transformers are typically exposed.
Accordingly, in yet a further embodiment, the present invention comprises a multiple transformer design of three or more similarly-sized transformers, each transformer comprising at least a primary winding and a secondary winding disposed on core means;
(a) said core means circumferentially disposed about an inner housing means;
(b) each transformer and inner housing member being materially encapsulated within an outer housing means, said outer housing means and said inner housing means together forming a common electrical shorted turn about each transformer;
(c) the secondary winding of each transformer being electrically coupled to an electrical load;
(d) each of said pair of transformers, when an electrical current is passed through said primary winding thereof, inducing an eddy current within said shorted turn; and
(e) the primary winding of each transformer electrically coupled to the primary winding of the other in a manner such as to cause the induced eddy currents from one transformer that flow around said shorted turn to be of a magnitude and direction so as to substantially cancel combined induced eddy currents from the other transformers.